<style>
div#css2 {
width: 100px;
border:1px solid;
} 

div#css3 {
width: 100px;
border:1px solid;
word-wrap: break-word;
} 


</style>
<h3>CSS2</h3>
<div id="css2">heeeeellang-woordje</div> 
<br /><br />

<h3>CSS3</h3>
<div id="css3">heeeeeellangwoordje</div>